Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 comprising a plurality of data tracks 4 defined by a number of servo sectors 60-6N recorded around the circumference of each data track. Each servo sector 6, comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a track address, used to position the head over a target data track during a seek operation. Each servo sector 6, further comprises groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a data track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
When reading data from the disk, a read channel typically samples the read signal to generate sample values that are equalized into a target response (e.g., a target partial response). A sequence detector detects an estimated data sequence from the equalized samples, and errors in the estimated data sequence are corrected, for example, using an iterative decoder such as a Turbo Code decoder, Low Density Parity Check (LDPC) decoder, or any suitable message passing decoder (a.k.a. belief propagation decoder).
It is desirable to measure the performance of the disk drive in terms of bit error rate in order to qualify each disk drive as acceptable and/or calibrate various parameters of each disk drive (e.g., by selecting a data density or calibrating read channel parameters). Since the bit error rate of an iterative decoder is typically very low, it is desirable to margin the read channel in order to expedite the qualification and/or the calibration procedures.